Jean-Marc M. Millet

Characterization of tellurium in MoVTeNbO catalysts for propane oxidation or ammoxidation

Abstract The oxidation state and local geometry of tellurium in MoVTeNbO catalysts used in the ammoxidation or oxidation of propane were characterized by X-ray absorption, Mossbauer, and X-ray photoelectron (XPS) spectroscopies. The results obtained by Mossbauer, and XPS spectroscopies showed that the catalysts contained Te(IV) in the bulk and mainly Te(VI) at the surface. X-ray absorption fine structure (EXAFS) measurements allowed to determine that the tellurite entities corresponded to TeO4E trigonal bipyramid in the hexagonal phase and to TeO3E somewhat distorted trigonal pyramid in the orthorombic one. The completely determined environment appeared to correspond in both phases to crystallographic sites in hexagonal channels. These results allowed to determine the stoichiometries of the two phases which are TeM3O10 for the hexagonal phase and Te2M20O57 for the orthorombic phase (M=Mo, V, Nb). 125Te Mossbauer isomer shift and pre-peak surface of the X-ray absorption spectra of the Te LIII-edge have been correlated and contributions of the Te(5s) and Te(2p3/2) to the structure have been analyzed.

FePO Catalysts for the Selective Oxidative Dehydrogenation of Isobutyric Acid into Methacrylic Acid

Abstract This review presents the iron phosphorus oxides used as catalysts for isobutyric acid oxidative dehydrogenation. Research on this catalytic system has been developed in the last decade and many publications have been devoted to this reaction, as it can be a step in a new process of production of methyl methacrylate. We emphasize particularly the nature of the active phase, the active centers, and the role of water and promoters. The mechanistic aspects of the reaction, which corresponds to an extension of the Mars and van Krevelen mechanism with a special role of water partial pressure, are discussed.

Effect of iron counter-ions on the redox properties of the Keggin-type molybdophosphoric heteropolyacid: Part I. An experimental study on isobutane oxidation catalysts

Abstract Keggin-type molybdophosphoric heteropolyacid with protons partially substituted by iron cations in a bulk form (Fe0.85H0.45PMo12O40) or supported on the cesium salt (Cs2Fe0.2H0.4PMo12O40) have been synthesized and characterized by different techniques like the Mossbauer spectroscopy and the electron spin resonance (ESR). The effect of iron on the redox and catalytic properties for the oxidation of isobutane into methacrylic acid (MAA) has also been studied. Iron has been shown to have a different effect whether acts as a counter-cation in the bulk acid or in the acid supported on the cesium salt. In the first case, it increases both the selectivity in methacrylic acid and methacrolein (MA) and the activity of the acid phase whereas in the second case, it increases only the selectivity. This difference has been explained by the existence of an electron transfer between iron and molybdenum occurring only in the bulk acid. This electron transfer was related to a combined hydration-oxidation mechanism which promotes the reducibility of the solid and consequently its catalytic activity.

Mössbauer spectroscopic study of iron phosphate catalysts used in selective oxidation

Three iron phosphate compounds FePO4, Fe2P2O7 and Fe7(PO4)6 have been synthesized and studied as catalysts for the oxidative dehydrogenation of isobutyric acid to methacrylic acid. Mössbauer spectroscopic characterization of the solids before and after catalytic reaction has allowed us to show that the starting phases were transformed in the conditions of reaction and that a new phase was formed. This new phase is a mixed Fe3+ and Fe2+ phosphate with a Fe3+/Fe2+ ratio of 1.8±0.2 with Mössbauer parameters: δ1=0.47 ± 0.05, Δ1=0.68± 0.02 mm.s-1 and δ2=1.20 ± 0.05, Δ2=2.73±0.02 mm.s-1. This new phase appears to be elective for methacrylic acid formation.

Effects of the method of preparing titanium pyrophosphate catalyst on the structure and catalytic activity in oxidative dehydrogenation of n-butane

Abstract Five different preparation methods have been developed to synthesize titanium pyrophosphate (TiP 2 O 7 ) which are active and selective catalysts for oxidative dehydrogenation of n -butane into butene and butadiene. The crystallinity, surface area, reducibility by n -butane and acid–base properties appeared different depending upon the method used. The activity of the catalysts in the oxidative dehydrogenation of n -butane appeared to depend mostly upon their redox properties whereas the selectivity was influenced by their acidic properties. Both properties seemed to be correlated to the crystallinity of the compounds but not to their surface area.

Semiconductive and Redox Properties of Ti and Zr Pyrophosphate Catalysts (TiP2O7 and ZrP2O7). Consequences for the Oxidative Dehydrogenation of n-Butane

The electrical conductivity of titanium and zirconium pyrophosphates used as catalysts in n-butane oxidative dehydrogenation has been measured under oxygen and n-butane at 400 and 500 °C and under subsequent exposures to both gases at the catalytic reaction temperature. The two compounds appeared to be p-type semiconductors under air with positive holes as the main charge carriers but became n-type when contacted with n-butane. If their conductivities are comparable as p-type semiconductors (within one order of magnitude), by contrast, they differ by 3 orders of magnitude when being n-type semiconductors. These results explain the difference in catalytic reaction mechanism encountered on the two solids. The alkane activation was proposed to be related in both cases to the p-type semiconducting properties of the solids, likely through hydrogen abstraction by a surface O- species, forming a C4H9• radical which will similarly undergo a second hydrogen abstraction to form butenes. The changes in activation energy and in selectivity on TiP2O7 at higher temperatures (>450 C) are indicative of a change in mechanism, possibly with the transient formation of an alkoxide species.

Kinetic study of isobutyric acid oxydehydrogenation on various FePO catalysts: Proposal for the reaction mechanism

Abstract A mechanism and rate expressions are proposed to explain the conversion kinetics of isobutyric acid to methacrylic acid by oxidative dehydrogenation on several iron phosphate catalysts. The relation between the catalyst nature and rate constants led us to suggest improvements in the catalysts and in reaction conditions. The mechanism proposed involves surface hydroxyl groups and sites composed of two nearby Fe cations. It corresponds to a new extension of the well-known Mars and Van Krevelen mechanism, often postulated for partial oxidation reactions on oxides.

Determination of the Oxidation State of Antimony, Iron and Vanadium in Mixed Vanadium and Iron Antimonate Oxide Catalysts

Vanadium and mixed vanadium and iron antimonates with rutile-type structures have been studied by XANES at Sb L1 edge, 57Fe Mössbauer spectroscopy and ESR spectroscopy at 77 K. The results showed that both antimony and iron remained at their highest oxidation state, i.e. SbV and FeIII, whereas vanadium was present as VIII and VIV. Two types of VIV species were distinguished corresponding to well-isolated vanadyl species in distorted octahedral coordination and vanadyl species in the same coordination but close to each other and in a dipole–dipole interaction. Both VIII and total VIV concentrations decreased when the iron content increased, whereas isolated VIV concentration increased first and then decreased, with a maximum for x = 0.2 in FexV1-xSbO4. The observed variations in cationic composition are discussed in relation with the catalytic properties of the compounds in the ammoxidation of propane. Isolated VIV-O moieties appeared to be the most active and selective catalytic sites.

Mössbauer study of synthetic oxidized vivianite at room temperature

Mössbauer spectroscopy has been used to study synthetic vivianites which are oxidized at room temperature in air. Six doublets, three ferrous and three ferric, have been used to fit the spectra recorded at 295 K. They have been attributed to the cations occupying the two different crystallographic sites. These sites were either isolated (I) or in pairs (II). In the case of the paired sites, two types of ferric cations and two types of ferrous cations can be distinguished, depending upon the degree of oxidation of the cation occupying the closest isolated site. Our experimental data showed that the ferrous cations occupying sites I were preferentially oxidized at the beginning of the oxidation process and that the rates of oxidation of the cations occupying two sites were comparable at a high oxidation level. We have also observed that the concentration of Fe3+ tends to a stabilized value of approximately 50% after 375 days, which also corresponds to the limit of stability of the vivianite structure.

Mössbauer Spectroscopic Study of Iron Containing Hydrotalcite Catalysts for the Reduction of Aromatic Nitro Compounds

Carbonated layered double magnesium hydroxides containing Fe with hydrotalcite structure which are active and selective catalysts for the reduction of aromatic nitro compounds by hydrazine hydrate have been prepared and characterized by X-ray diffraction and Mössbauer spectroscopy. Using these techniques we have shown that when the Fe/(Fe + Mg) ratio was greater than 0.2, which corresponds to the natural phase composition, ferrihydrite Fe5HO8⋅4H2O was formed. This last phase, characterized at room temperature by a superparamagnetic doublet, leads after activation under nitrogen at 723 K to small particles of ferric oxide. An in-situ study of the activated catalysts allowed us to show that only iron cations present in this phase could undergo an oxido-reduction under the conditions of catalysis and that the catalytic properties must be related to this last phase.